I am a low voltage XLPE compounder. I am running black and natural color for the compound. Every time when my production machine swings from black to natural compound, I have to use out about 300 - 400kg of virgin material to purge the black/ contaminated. Even though the natural compound looks white, there are still some contaminants in the products.
Can I ask if anyone has a remedy for this? Your help will be very much appreciated.
There are certain facts of life that are hard to get around and I expect this is one of them. Since you are manufacturing low voltage XLPE, I imagine you are purging the compounding extruder with natural polyethylene containing no organo-peroxides at all. This suggests that there is no or very little peroxide bearing polyethylene in the purged compound once the majority of the compounded black XLPE polyethylene is captured and discarded. It may then be possible to capture and regrind the remaining "gray" polyethylene in say a hot melt granulator and that this material could then be proportionally fed into the next run of the black XLPE formulation. The danger of course is that some larger concentrations of peroxide bearing polyethylene could go through the compounding extruder a second time and result in precured or already crosslinked contaminants in the finished compound. Other contaminants such as plant dirt and foreign materials could also be a problem. This will depend on operator/ process engineering expertise, the sophistication of your compounding extruder, plant cleanliness and local specifications. You may wish to discuss this further with the compounding extruder manufacturer.
I have faced the same problem as well. I have been using virgin material to clean up the compounding extruder for years. I have been told that I can either add in purging compound or processing aids (PPA) to do the cleaning. I would like to ask what are the difference and advantages between purging compound and processing aids in terms of the quality of the product."
I am neither a chemist nor chemical engineer so the following are strictly my personal opinions:
"Purging Compounds and Polymer Processing Aids (PPA) are just one more material to purchase and use in the hope that one will get a faster clean out and thus use less material. My experience with extruders that pump thermosetting and thermoplastic polyolefin compounds is that these extras are not justifiable in terms of saving money. Moreover, the common PPA compounds on the market are fluorinated, expensive and environmentally harmful. Instead I suggest that one use a natural polyethylene thermoplastic compound to clean out the extruder but continuously vary the extruder speed to "scrub" the screw and barrel so to speak.
Others may have different opinions and I would very much appreciate if they are posted on this Forums thread.
Extracts from a Presentation Titles - Processing Aids in Extrusion of Polyethylene
By: M.H. Wagner (1), M. Müller (1), O. Kulikow (2), K. Hornung (2)
(1) - Technische Universität Berlin, Polymertechnik/-physik
(2) - Universität der Bundeswehr der München, Luft- und Raumfahrttechnik
Polymer Processing Aids (PPAs) are often used improve extrusion flow instabilities in LLDPE and metallocene PE such as melt fracture and this is often referred to as "snakeskin".
Metallocene Polyethylene (mPE)
• mPE is a new development which is of special interest because of narrow molar mass distribution:
• Polyethylene made from metallocene catalysts en.wikipedia.org/wiki/Post-metallocene_catalyst is tougher, stronger and cleaner than resin made from conventional catalysts
• The film manufactured with metallocene PE offers down-gauging opportunities to reduce costs and environmental pollution